The primary auditory cortex (AI) in the cat will be studied using a variety of neuroanatomical techniques. Chief among these is the Golgi method, which will be used to analyze the structure of the varieties of principal cells and interneurons, and to correlate the disposition of the morphologically defined classes of cells with the patterns of afferent endings. These extrinsic axons originate in the thalamus, in the contralateral cerebral cortex, in the ipsilateral cortex adjacent to and distant from AI, and from within AI. The shape, structure and terminal ramification of each type of axon will be described in sufficient detail, along with the morphology of the cells, to compare the organization of AI with that of the primary visual and somatic sensory cortices. In this way, it should be possible to determine if a common plan of organization, with certain variations, accounts for the structure of primary sensory cortex, or if idiomatic principles apply to each primary sensory field. In addition to the morphological studies of nerve cells and axons described above, the laminar architecture will also be studied, and the terminal zones of callosal, cortico-cortical and thalamic input identified and characterized. In this way, the neurons which may be postsynaptic to the various terminals can be described and a synthetic picture of the sequence and flow of cortical information built up. This, in turn, will be used as a framework to study the vertical and radial organization of AI, and to define some potential substrates for columnar organization, frequency representation, summation and suppression zones and other physiological properties of neurons in AI. Beside the Golgi studies, a small number of connectional experiments will identify the cells and nuclei of origin of certain axon populations described in the morphological investigations. The pattern of afferent and efferent connections in AI, the laminar distribution of axon terminals, and the varieties of neurons will provide a framework for future studies of fine structure as well as investigations of the anatomy of nonprimary auditory cortex.